These cactus are very forgiving but they still have 2 requirements: full sun and pots with drainage. Its not about how fast the soil dries or how little you water. Its all about how much oxygen the roots are getting. Pull it out of that pot and put it into a proper draining pot and it will grow rapidly and happily.
I have 4 - I must like them. Your lack of humidity is only good for keeping the mirrors from steaming over when you shower. Your cactus still needs proper drainage and proper watering.
